Maternal behavior in the rabbit is unusual among mammals. Shortly after parturition the doe leaves the litter and returns around 24 hours later to nurse their pups during 3-5 minutes every day. Chronobiological studies demonstrates that the suckling cycle of the pup is a circadian rhythm as they shows food anticipatory activity before the arrival of the mother. This animal model represents an extraordinary opportunity to study the ontogeny of circadian rhytms with minimal disturbance of the dyad, as mother and young come together once a day for a period lasting only a few minutes. The principal objective of this proposal is to study the anatomy and physiological development of the Suprachiasmatic Nucleus (SCN) of the rabbit when the mother is the principal zeitgeber for the pups. Our final goal is to look for changes in the SCN and extra SCN-forebrain structures in response to changes in timing of the nursing bout and to compare with those induced by light considering that first the mother is the principal zeitgeber and then there is a shift to a light controlled oscillator. The specific aims are: I: Neurogenesis in the Suprachiasmatic Nucleus II: Development of the Retinohypothalamic tract of rabbit pups III: To determine the ontogeny of a photic response in the suprachiasmatic nucleus by ligth-induction of the FOS protein in the SCN and to determine the peptidergic identity of the light-induced cells. IV: To determine the ontogeny of expression of PER1, PER2 and PER3 proteins in the SCN and extra-SCN forebrain structures at PD1, PD7, PD15 and PD20 in LID condition and in response to a light pulse. For all Aims we will use immunocytochemical techniques with specific antibodies. For Aim I we will inject BrdU i.p. to pregnant does from embryonic day 9-30. Pups will be sacrificed at PD1 to determine the day of birth of cells. For Aim II we will inject Cholera Toxin B in the humor vitreous and subjects will be sacrificed 48 h later. For aim III pups will be exposed for 30 min to a light pulse (800 lux) during subjective night and will be sacrificed 60 min later. For Aim IV pups will be scheduled to suckle at different time points, sacrificed every 4 hours around time of suckling in L/D and after a light pulse during subjective night. In this aim the hypothesis is that there is a shift from a food contolled circadian rhythmicity to a light controlled circadian rhythmicity. This research aims to identify and characterize pacemaker cells of the SCN and other brain regions that regulate circadian disorders, e.g. jet lag, shift work. Once characterized, we can use this information to control circadian rhythms in circumstances where it is disrupted.